1. Field of the Invention
The present invention relates to the art of preventing a core from being broken during dewaxing or production of a wax pattern to be used in a lost wax casting process for casting a hollow part by using the core.
2. Description of the Prior Art
Castings having hollow portions, such as blades for gas turbines, have cooling holes of complicated shapes in their interiors. A lost wax casting process is widely used for producing such castings, and in that process, it is necessary to produce a wax pattern having a core in advance. The wax pattern is produced by incorporating a ceramic core into wax, and the conventional producing method is normally performed as follows.
FIG. 2 is a plan view of a ceramic core 1, and FIG. 3 is its II--II cross-sectional view. The core 1 has a multiplicity of complicated portions such as through-holes 2, slits 3 and a thin portion 4, and their surrounding portions are very brittle. This core 1 is incorporated into a die 5 as shown in FIG. 5. The die 5 has a cavity 6 having the same shape as the external shape of a casting, and the top side of the die 5 has a nozzle opening 7 through which to inject a wax material. The wax material is normally injected into the cavity 6 through the nozzle opening 7 by an injection machine. The injection pressure is set to approximately ten to several tens of kgf/cm.sup.2. The cavity 6 and the through-holes 2 and the slits 3 of the core 1 are charged with the wax material injected into the die 5, and a wax pattern in which the core 1 is coated with wax is formed by cooling and solidifying the wax material.
A plurality of wax patterns each having the above-described form are assembled into a wax cluster. A mold is formed to surround this wax cluster, by investment process or ceramic shell molding process. The wax embedded in the mold (corresponding to the cavity 6) is melted away, excluding the core 1, and the mold is baked at temperatures of 900 to 1,500.degree. C. and molten metal is poured into a cavity formed in the mold after the wax has been melted away. After the solidification of the molten metal, a casting which has the core 1 in its interior is obtained by breaking away the mold and separating the casting cluster. Finally, the casting is dipped into an alkali solution to melt away the remaining ceramic core 1 in the interior of the casting, thereby preparing a casting having a hollow portion corresponding to the shape of the core 1.
In the above-described conventional method, since the surrounding portions of the thin portion 4, the through-holes 2 and the slits 3 in the core 1 are very brittle, they are very frequently broken by high pressure during the injection of the wax material. After the wax material is injected, the core is broken with high frequency by stress due to the solidification of the wax material. Furthermore, a brittler portion of the core 1 is broken earlier with high frequency by the outflow stress of the wax during dewaxing.